Sanitize filename prior to creating contact files

[quinn-p-mchugh]

Some of my Google Contacts contain characters that are incompatible with Windows file systems, such as <>:"/\\|?*. This caused an error when trying to create markdown files for these contacts.

This pull request adds a method to the md_person class called _sanitize_filename and uses that method to replace these invalid characters with underscores before markdown files are created. This prevents errors with trying to create files with invalid paths.